Break right foot when crossing isle (4)
I believe the answer is:
rift
'break' is the definition.
(rift can mean a break or opening)
'right foot when crossing isle' is the wordplay.
'right' becomes 'r' (abbreviation).
'foot' becomes 'ft' (abbreviation).
'when crossing' indicates putting letters inside.
'isle' becomes 'i' (geographical abbreviation).
'r'+'ft'='rft'
'rft' enclosing 'i' is 'RIFT'.
(Other definitions for rift that I've seen before include "Schism (in the lute?)" , "Lack of harmony" , "cleavage" , "Cleft in earth or breach in relationship (4)" , "Cleft in rock" .)